If you buy a "stainless steel" boat fitting without asking which grade, you are probably getting AISI 304. It is cheaper, easier to machine, and "stainless" enough to pass a visual test on the showroom floor. In salt water, 304 is basically a time bomb. Understanding the difference takes two minutes and saves a lot of ruined hardware.
Both alloys are austenitic stainless steels with ~18% chromium and ~8-10% nickel. The critical difference is molybdenum. AISI 316 contains 2-3% molybdenum. AISI 304 has essentially none. That 2% changes everything about how the steel behaves when chloride ions (salt) attack the passive chromium-oxide layer that gives stainless its corrosion resistance.
On 304 in salt spray, the passive layer breaks down at localized points — especially at crevices, sharp edges, and stress concentrations. You get pitting corrosion: tiny black dots within weeks, brown rust streaks in 3-6 months, structural pitting in 12-18 months. It looks like the steel is bleeding. This is why cheap stainless railings and rod holders go rusty on dockside boats so fast.
On 316, the molybdenum stabilizes the passive layer even in aggressive chloride environments. Pitting resistance goes up by roughly 10x. A 316 fitting that sees daily salt exposure can look clean for a decade with occasional rinse-and-wipe maintenance. This is why all structural marine hardware — cleats, stanchions, bow rails — is specified in 316 on any boat built for offshore use.
316L (low-carbon variant, ≤0.03% C) is what MARINAC uses for all welded marine parts. The "L" prevents carbide precipitation at grain boundaries during welding, which otherwise creates corrosion paths at the weld zone. For bolt-together assemblies, standard 316 (0.08% C max) is fine and slightly cheaper.
How to verify what you have. The simplest field test: apply a drop of nickel sulphate + hydrochloric acid (Mo-test reagent, available online). 316 turns dark blue-green within 30 seconds; 304 stays colorless. Less destructive: check the product page for a material cert — "316" or "A4" (European equivalent) for real marine-grade; "304" or "A2" for freshwater-only or indoor use.
Why some suppliers save on grade: at our purchase volume, 316 rod costs about 35-40% more than 304 per kilogram. On a single fitting it's €2-5 difference. Scale to 10,000 units and it becomes real money. Some manufacturers absorb the premium, others don't — and the customer finds out 18 months later.
MARINAC rule: every product that touches salt water is 316 or better. Every product page lists the grade. If a competitor's page just says "stainless steel" without specifying — assume 304. The price might look identical; the lifetime is not.